As semiconductor devices have developed there has been a considerable increase in their operational speed, and a reduction in overall size. This is causing a major problem of heat build-up in the semiconductor devices. Therefore, heat sinks are being used to help dissipate the heat from the semiconductor device. Such heat sinks are normally fabricated separately from the semiconductor device and are normally adhered to the semiconductor device just prior to encapsulation.
There have been many proposals for the electroplating of copper onto surfaces of semiconductor devices during their fabrication, particularly for use as interconnects.
The majority of current semiconductor devices are made from semiconductor materials based on silicon (Si), gallium arsenide (GaAs), and indium phosphide (InP). Compared to such electronic and optoelectronic devices, GaN devices have many advantages. The major intrinsic advantages that GaN have are summarised in Table 1:
TABLE 1Band GapBFOM(eV)/(powerMaximumMobility μwavelengthtransistorTemperatureSemiconductor(cm2/Vs)(nm)merit)(C.)Si1300 1.1/11271.0300GaAs50001.4/8869.6300GaN15003.4/36024.6700
From Table 1, it can be seen that GaN has the highest band gap (3.4 eV) among the given semiconductors. Thus, it is called a wide band gap semiconductor. Consequently, electronic devices made of GaN operate at much higher power than Si and GaAs and InP devices.
For semiconductor lasers, GaN lasers have a relatively short wavelength. If such lasers are used for optical data storage, the shorter wavelength may lead to a higher capacity. GaAs lasers are used for the manufacture of CD-ROMs with a capacity of about 670 MB/disk. AlGaInP lasers (also based on GaAs) are used for the latest DVD players with a capacity of about 4.7 GB/disk. GaN lasers in the next-generabon DVD players may have a capacity of 26 GB/disk.
GaN devices are made from GaN wafers that are typically multiple GaN-related epitaxial layers deposited on a sapphire substrate. The sapphire substrate is usually two inches in diameter and acts as the growth template for the epitaxial layers. Due to lattice mismatch between GaN-related materials (epitaxial films) and sapphire, defects are generated in the epitaxial layers. Such defects cause serious problems for GaN lasers and transistors and, to a lesser extent, for GaN LEDs.
There are two major methods of growing epitaxial wafers: molecular beam epitaxy (MBE), and metal organic chemical vapour deposition (MOCVD). Both are widely used.
Conventional fabrication processes usually include these major steps: photolithography, etching, dielectric film deposition, metallization, bond pad formation, wafer inspection/testing, wafer thinning, wafer dicing, chip bonding to packages, wire bonding and reliability testing.
Once the processes for making LEDs are completed at the full wafer scale, it is then necessary to break the wafer into individual LED chips or dice. For GaN wafers grown on sapphire substrates, this “dicing” operation is a major problem as sapphire is very hard. The sapphire first has to be thinned uniformly from about 400 microns to about 100 microns. The thinned wafer is then diced by diamond scriber, sawed by a diamond saw or by laser grooving, followed by scribing with diamond scribers. Such processes limit throughput, cause yield problems and consume expensive diamond scribers/saws.
Known LED chips grown on sapphire substrates require two wire bonds on top of the chip. This is necessary because sapphire is an electrical insulator and current conduction through the 100-micron thickness is not possible. Since each wire bond pad takes about 10-15% of the wafer area, the second wire bond reduces the number of chips per wafer by about 10-15% as compared to single-wire bond LEDs grown on conducting substrates. Almost all non-GaN LEDs are grown on conducting substrates and use one wire bond. For packaging companies, two wire bonding reduces packaging yield, requires modification of one-wire bonding processes, reduces the useful area of the chip, and complicates the wire bonding process and thus lowers packaging yield.
Sapphire is not a good thermal conductor. For example, its thermal conductivity at 300 K (room temperature) is 40 W/Km. This is much smaller than copper's thermal conductivity of 380 W/Km. If the LED chip is bonded to its package at the sapphire interface, the heat generated in the active region of the device must flow through 3 to 4 microns of GaN and 100 microns of sapphire to reach the package/heat sink. As a consequence, the chip will run hot affecting both performance and reliability.
For GaN LEDs on sapphire, the active region where light is generated is about 3-4 micron from the sapphire substrate.